On your 13 x 19 paper, print the image at 12x18 for mounting in an 18x24 frame. That way, you'll have some white area that underlays your mat. When you cut the window in your mat to 11.75 x 17.75 inches (overlapping your image by 1/8 inch on all sides), you'll have a mat width of (about) 3 inches on all sides. Looks great. Of course, if you're lazy like me, print the image at 12.25 x 18.25 inches, then you can eliminate the fancy math and cut the window in the mat to exactly 12x18.